The purpose of this study is to determine whether psoralen-longwave ultraviolet light (PUVA) therapy to patients with psoriasis will cause deleterious effects such as aging and cancer of the skin. A clinical and laboratory study will include one hundred patients to be evaluated for a 3 - 5 year period. To determine whether aging and cancer of the skin will be consequences of PUVA therapy we plan to examine the morphological and biochemical parameters in the psoriatic plaque and in uninvolved skin of the patients. Using morphological criteria, we will determine the temporal sequence of tissue changes in biopsy material. These studies will be complemented by tissue culture assays designed to detect hazards of this treatment. Using a mammalian cell line, we will examine the toxicity and transforming potential of this type of photochemotherapy in vitro. In addition we will identify the photoproducts of irradiated 8-methoxypsoralen (8-MOP) in vivo and in vitro. We will examine the biochemical mechanisms and morphological changes of the psoriatic plaque associated with its regression after PUVA therapy and will attempt to explain variations in response to the therapy. Procedures to be employed will include chemical analyses of 8-MOP in serum, measurements of DNA synthesis and repair by autoradiography, light and electron microscopic examinations, and study of kinetics of binding and release of photoactivated 8-MOP in cultured fibroblasts.